U.S. patent number 4,957,471 [Application Number 07/383,630] was granted by the patent office on 1990-09-18 for adjustable locked center and dynamic tensioner.
Invention is credited to Richard C. St. John.
United States Patent |
4,957,471 |
St. John |
September 18, 1990 |
Adjustable locked center and dynamic tensioner
Abstract
An adjustable locked center and dynamic tensioner include both a
method for setting tension in a belt or chain drive system and
apparatus for setting the friction torque so as to match the system
setting torque. The method includes forcing the tensioner against a
stable mounting surface with sufficient spring load that a target
friction torque will be required to rotate the tensioner. The
tensioner arm is then rotated into the belt or chain until, when
the setting torque is removed, the belt or chain will counter
rotate the tensioner arm and the target friction torque in the
tensioner will cause the target setting tension to remain in the
system. The apparatus includes a tensioner arm pivotally mounted on
a fixed surface with a spring engaging the arm and being clamped
against the fixed surface. Modifications include introducing a
compliant coupler such as an elastomeric bushing or spring between
the stable mounting surface and the tensioner arm.
Inventors: |
St. John; Richard C. (North
Canton, OH) |
Family
ID: |
23514000 |
Appl.
No.: |
07/383,630 |
Filed: |
July 24, 1989 |
Current U.S.
Class: |
474/133;
474/135 |
Current CPC
Class: |
F16H
7/1281 (20130101); F16H 2007/081 (20130101); F16H
2007/088 (20130101); F16H 2007/0893 (20130101) |
Current International
Class: |
F16H
7/12 (20060101); F16H 7/08 (20060101); F16H
007/08 () |
Field of
Search: |
;474/101,109-111,113-117,133,135 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2608277 |
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Sep 1977 |
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DE |
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3043287 |
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Jun 1981 |
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DE |
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279415 |
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Mar 1952 |
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CH |
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Primary Examiner: Bui; Thuy M.
Attorney, Agent or Firm: Taylor; Reese
Claims
What is claimed is:
1. A method of setting tension in a belt or chain drive system
using a pivoting belt tensioner, comprising the steps of:
(a) forcing the tensioner against a stable mounting surface with a
clamping load of such a magnitude that a target friction torque
will be required to rotate the tensioner; and
(b) rotating the tensioner arm against the belt or chain until,
when the setting torque is removed, the belt counter-rotates the
tensioner arm and the target friction torque causes the target
tension to remain in the belt.
2. The method of claim 1 further characterized by inserting a
compliant coupler between the stable mounting surface and the
tensioner to accommodate tension surges in the belt or chain drive
system.
3. A method of setting tension in a belt or chain drive system
using a pivoting belt tensioner, comprising the steps of:
(a) forcing the tensioner against a stable mounting surface with a
clamping load of such a magnitude that a target friction torque
will be required to rotate the tensioner; and
(b) rotating the tensioner arm against the belt or chain with a
torque applying and measuring device to a predetermined level
greater than the target friction torque; and
(c) locking the tensioner against counter rotation to achieve a
predicted target tension.
4. The method of claim 3 further characterized by inserting a
compliant coupler between the stable mounting surface and the
tensioner to accommodate tension surges in the belt or chain drive
system.
5. A tensioner for setting tension in a belt or chain drive system
and adapted for mounting on a stable portion of the drive system,
comprising:
(a) an elongate tensioner arm having an arcuate slot in one
end;
(b) a pulley received on the opposed end of said arm;
(c) a pivot engagable with said arm and the stable portion of the
drive system;
(d) friction means disposed about said pivot and in engagement with
said arm, said friction means being compressible to apply clamping
force between said arm and the stable portion of the drive system;
and
(e) locking means for locking said arm relative to the stable
portion of the drive system.
6. The tensioner of claim 5 wherein a pointer is disposed on said
arm.
7. The tensioner of claim 6 wherein said pivot comprises a pivot
screw having first and second ends; said locking means comprise a
locking nut received on said second end of said pivot screw and
forcing said friction means into engagement with the stable portion
of said drive system; said pointer comprises an elongate plate
lying on said arm; a washer is received about said pivot screw
adjacent said second end thereof; said plate of said pointer having
a through aperture such that said washer is receivable therein and
said washer being thicker than said plate whereby said pivot screw
bears on said washer when said locking nut is tightened.
8. The tensioner of claim 6 wherein a lock screw is provided for
engagement with said pointer and the stable portion of the drive
system.
9. The tensioner of claim 5 wherein said pivot comprises a pivot
screw having first and second ends; and said locking means comprise
a locking nut received on said second end of said pivot screw and
forcing said friction means into engagement with the stable portion
of said drive system.
10. The tensioner of claim 5 wherein a spacer is disposed between
said tensioner arm and the stable portion of the drive system.
11. The tensioner of claim 5 wherein said pivot is knurled for
engagement with said tensioner arm.
12. The tensioner of claim 5 wherein said friction means include a
spring.
13. A tensioner for setting tension in a belt or chain drive system
and adapted for mounting on a stable portion of the drive system,
comprising:
(a) an elongate tensioner arm having an arcuate slot in one
end;
(b) a pulley received on the opposed end of said arm;
(c) a pivot engagable with said arm and the stable portion of the
drive system;
(d) friction means disposed about said pivot and in engagement with
said arm, said friction means being compressible to apply clamping
force between said arm and the stable portion of the drive
system;
(e) locking means for locking said arm relative to the stable
portion of the drive system;
(f) a torque plate received on the stable portion of the drive
system; and
(g) a lock screw releasably securing said torque plate to the
stable portion of the drive system.
14. The tensioner of claim 13 wherein said lock screw passes
through said arcuate slot; and a spacer is received within said
arcuate slot between the head of said lock screw and said torque
plate; and said spacer being freely received with respect to the
edges of said arcuate slot.
15. The tensioner of claim 13 wherein compliant means are disposed
about said pivot.
16. The tensioner of claim 15 wherein said compliant means include
an elastomeric bushing.
17. The tensioner of claim 16 wherein said elastomeric bushing
includes concentric outer and inner sleeves and an elastomeric
sleeve therebetween.
18. The tensioner of claim 15 wherein said pivot is knurled for
engagement with said compliant means.
19. The tensioner of claim 13 wherein said pivot comprises a pivot
screw having first and second ends; and said locking means comprise
a locking nut received on said second end of said pivot screw and
forcing said friction means into engagement with the stable portion
of the drive system.
20. The tensioner of claim 13 wherein said friction means include a
spring.
21. The tensioner of claim 13 wherein one end of said tensioner arm
carries angle indicia.
22. A tensioner for setting tension in a belt or chain drive system
and adapted for mounting on a stable portion of the drive system,
comprising:
(a) an elongate tensioner arm having first and second ends;
(b) a pulley received on said first end of said arm;
(c) a pivot engagable with said second end of said arm and the
stable portion of the drive system;
(d) friction means disposed about said pivot and in engagement with
said arm, said friction means being compressible to apply clamping
force between said arm and the stable portion of the drive
system;
(e) locking means for locking said arm relative to the stable
portion of the drive system; and
(f) a torque plate disposed between said tensioner arm and the
stable portion of the drive system.
23. The tensioner of claim 22 wherein compliant means are disposed
about said pivot.
24. The tensioner of claim 23 wherein said compliant means include
an elastomeric bushing.
25. The tensioner of claim 24 wherein said bushing includes
concentric inner and outer sleeves and an elastomeric sleeve
therebetween.
26. The tensioner of claim 23 wherein said pivot is knurled for
engagement with said compliant means.
27. The tensioner of claim 22 wherein said pivot comprises a pivot
screw having first and second ends; and said locking means comprise
a locking nut received on said second end and forcing said friction
means into engagement with the stable portion of the drive
system.
28. The tensioner of claim 22 wherein said friction means include a
spring.
Description
BACKGROUND OF THE INVENTION
This invention relates, in general, to belt and chain drive
tensioners and relates, in particular, to a belt or chain drive
tensioner intended to provide for the adjustment of tension in
serpentine drive systems.
DESCRIPTION OF THE PRIOR ART
It is known, from St. John U.S. Pat. No. 4,557,709 and others, that
power transmission drives, such as serpentine drives, generally
require a tensioning device to operate effectively and that a
properly positioned pivotal arm, spring-actuated, automatic
tensioner will provide functionally constant tension over a broad
enough range of operation to last the life of the drive system.
It is also known from the prior art, such as St. John U.S. Pat. No.
4,767,383, that an adjustable tensioner can be employed for much
the same purpose, except that periodic adjustments need to be made
to compensate for variations in belt length caused by original belt
and equipment manufacturing tolerances and belt stretch during use.
This art teaches that torque applied about the arm pivot point will
cause tension in the belt and that there are well-identified
mathematical relationships between the tensioner torque and the
belt tension. It has also been demonstrated that practically
constant tension can be provided over useful ranges of belt
elongation by constant torque applied about the pivot of the
tensioner when the tensioner is properly placed in the drive system
or, in other words, oriented correctly between the two pulleys that
straddle the tensioner.
Classic engineering principles also teach that there will be no
fundamental operational difference between a tensioner that employs
torque about the pivot to generate the belt tension or one that
uses belt tension to generate the torque about the pivot. In other
words, it is immaterial whether the tension component is the result
of the torque component, or whether the torque component is the
result of the tension component.
Automatic tensioners, such as shown in St. John U.S. Pat. No.
4,557,709 and other automatic tensioners, as well as the adjustable
tensioner of St. John U.S. Pat. No. 4,767,383, exploit the torque
to cause the tension. The present invention, however, exploits the
tension acting against the idler pulley of the system to cause the
torque.
SUMMARY OF THE INVENTION
It is, accordingly, a principal object of this invention to provide
an adjustable tensioner using friction to provide the predetermined
torque which matches the system requirements for target tension at
any point or over any specified range with specified nominal
accuracy.
It is a further object of the invention to use an undefined amount
of tensioner spring back after the setting torque is removed as an
indication that enough setting torque has been provided and thus
permit precise results to be derived from imprecise inputs without
the use of measuring devices.
It is a still further object of the invention to permit a friction
set device to be initially over-tensioned to compensate for initial
excessive belt stretch, but still provide the correct tensioning
torque in subsequent tensioning settings.
It is a still further object of the invention to utilize the
combination of predetermined and preset tension in conjunction with
a compliant torsion bushing to provide a superior and predictable
dynamic tensioner.
In accomplishing these objectives, friction means are employed
adjacent to the pivot screw of the tensioner to achieve the
predetermined torque. It will be noted that the pivot screw is
fixed or coupled to the tensioner arm, but free to rotate with
respect to the user mount. In essence, a locking nut on the pivot
screw is tightened against a member, such as a Belleville spring,
until the friction forces between the spring and the nut plus those
between the tensioner arm and the user mount are of the correct
magnitude to require a given torque value to be applied to the
pivot screw to cause the tensioner arm to rotate when it is
unconstrained by the belt or any other influence other than the
friction torque.
Accordingly, production of an improved adjustable locked center and
dynamic tensioner of the character above-described becomes the
principal object of this invention with other objects thereof
becoming more apparent upon a reading of the following brief
specification considered and interpreted in view of the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view showing one form of the friction set,
adjustable tensioner.
FIG. 2 is an elevational view, partially in section, showing the
tensioner of FIG. 1.
FIG. 3 is a plan view of a friction set, dynamic adjustable
tensioner.
FIG. 4 is an elevational view, partially in section, of the
tensioner of FIG. 3.
FIG. 5 is an elevational view, partially in section, of a further
modified form of an adjustable dynamic tensioner.
FIG. 6 is an elevational view, partially in plan, showing a still
further modified form of the dynamic adjustable tensioner.
FIG. 7 is an elevational view of a still further modified form of
the invention.
FIG. 8 is a partial perspective view of the form of the invention
illustrated in FIG. 7.
DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENTS
Before describing the various embodiments of the present invention
in detail, it should first be noted that, in subsequent embodiments
after the first embodiment illustrated in FIGS. 1 and 2 of the
drawings, similar reference numerals are utilized for similar
components, except that the numbers are in the 100,200, etc.,
series.
Referring then to FIGS. 1 and 2 of the drawings, it will be seen
that the tensioner, generally indicated by the numeral 10, includes
a tensioner arm 11, one end of which carries a pulley and bearing
assembly 12 mounted thereon by pulley screw 13. It should be noted
that the pulley illustrated is of the "flat" type, but many other
pulley styles are contemplated.
The opposed end of arm 11 is mounted on user mount 20 which, in
practice, can be any stable portion of the drive system, such as an
engine block. There are two means for attachment involved for
mounting tensioner 10 lock screw 30 and pivot screw 40, as will be
described below.
In that regard, a pointer 15 is disposed on the top of arm 11 and
lock screw 30 and its associated lock washer 31 are disposed on one
end thereof. Lock screw 30 is passed through a suitable aperture in
pointer 15, an arcuate slot 11a in arm 11 and is received in a
threaded bore 21 in mount 20, as can be seen in FIG. 2.
To enhance performance an enlarged aperture is provided in pointer
15 and a flat washer 32 is inserted in the bore so that the pivot
screw clamps on the washer 32 instead of on the pointer 15, as
shown in FIG. 2. A flat spacer 11b between the tensioner arm 11 and
the user mount 20 can also optionally be employed as illustrated
and has been found to be an effective means of insuring constant
friction torque when the user mount is rough, wavy or
unpredictable.
Pivot screw 40 also passes through suitable apertures or bores, in
pointer 15, arm 11 and user mount 20 and projects beyond the bottom
surface 20a of user mount 20. The projecting end of pivot screw 40
receives a Belleville spring 42 and a clamping nut 41. The shank of
pivot screw 40 is knurled at 40a to key the pivot screw to arm 11.
It should be noted that, while a Belleville spring is illustrated
and described and is a logical choice in terms of size, cost and
ease of mounting, other suitable force means, such as a coil spring
or elastomeric pad, could provide the necessary friction
torque.
The method of providing the setting torque in FIGS. 1 and 2 is to
force the tensioner arm 11 against the user mount 20 with a spring
load. When the spring load is of the proper value, the friction
torque between the tensioner arm 11 and either the user mount 20 or
spacer 11b, if one is used, will be of the proper value to provide
the desired static tension in the belt B when the belt acts against
the pulley and bearing assembly 12. Once the proper friction value
has been established, the static tension is set by rotating the
tensioner arm 11 into the belt hard enough so that, when the
setting torque is removed, the belt counter-rotates the tensioner.
When this backward motion occurs, no matter how slight it may be,
the belt will experience the tension that is associated with the
friction torque.
Setting the friction torque is generally a one-time operation, but
the belt tension may be set many times from the single friction
adjustment.
If the torque is applied to the tensioner arm 11 about the pivot
screw 40, the minimum value of applied torque to cause impending
backward motion will be twice the friction torque. Additional
applied torque will simply cause more belt stretch and more spring
back after the setting torque is removed, but the retained torque
will be the value of the friction torque and the tension in the
belt will be the target tension. In actual use, dynamic belt
tensions vary from the static tension set into the belt by the
means just described. The variation in dynamic tension could cause
unwanted motion of the tensioner arm 11 and subsequent loss in
static tension if means to prevent this motion were not provided by
the lock screw 30.
In operation, the lock screw 30 is tightened to clamp the tensioner
arm 11 against the user mount 20 or spacer 11b with a friction
torque that is much greater than the setting torque and of a value
high enough to resist any torque caused by excursions of dynamic
tension.
The method of providing the proper friction torque has been shown
as a Belleville spring 42 placed between the fixed surface, such as
the user mount 20, and the clamping nut 41. The clamping nut 41
clamps the Belleville spring 42 so that the spring is compressed
between the clamping nut 41 and the user mount 20. Rotation of the
clamping nut 41 increases or lessens the compression, depending
upon the direction of rotation, so judicious rotation of the nut
and selection of components can provide any amount of clamping
force between friction surfaces. The infinitely variable value of
friction force results in infinitely variable friction torque of
the adjustable tensioner, so any value of setting torque and
consequently belt tension can be had by the means just
described.
There are many variations of the friction surfaces and load
applying means. For example, a conventional helical compression
spring could replace the Belleville spring, the spring compression
could be governed by a snap ring with or without shims for
adjustment, and a friction disc or multiple friction discs could be
placed between the tensioner arm 11 and the user mount 20 in the
manner of power transmission clutches. As shown, most of the
friction torque is generated between the tensioner arm 11 and the
user mount 20 or spacer 11b, although some is generated between the
clamping nut 41 and the Belleville spring 42 and the user mount
20.
The theory of providing essentially constant belt tension with a
constant torque applied about the pivot screw 40 has been disclosed
in U.S. Pat. No. 4,767,383. This same theory applies to setting the
friction torque in the friction set adjustable tensioner, i.e., the
friction torque must be measured by applying torque about the pivot
screw 40. However, once that torque has been so found to be of a
satisfactory value, proper tension will be achieved by applying a
load to any place on the tensioner. The criteria for successful
tensioning are that the tensioner spring back by action of the belt
after the tensioning torque is removed and that the tensioning
torque not be so great as to unduly damage the belt or other system
components.
Although the primary goal of this device is to provide target
tension without having to make measurements after the initial
friction setting, there are other valuable ways in which the
tensioner may be used. One of these is that the friction provision
may be used as a biasing means to ease holding net tensioning
torque when the friction set device is used as an adjustable
tensioner. This is done by measuring the friction torque, adding
the tensioning torque to the friction value, applying that torque
sum to the pivot screw 40 and then tightening the lock screw 30.
The net torque will be the tensioning torque, and the tension in
the belt will be that provided by the tensioning torque. If the
friction torque is greater than the tensioning torque, the torque
wrench may be removed prior to tightening the lock screw 30. If the
friction torque is less than the tensioning torque, torque must be
held on the pivot screw 40, while the lock screw 30 is being
tightened. However, excursions in the holding torque on the pivot
screw 40 that are less than the friction torque will not result in
any variation in the net tensioning torque or the tension in the
belt. Consequently, even serious reductions from the ideal will not
impair tension once the proper tensioning torque has been
reached.
Most belt manufacturers recommend that the initial belt tension be
adjusted to between 50% and 100% greater than the normal operating
tension. The high initial tension is used because new belts stretch
very rapidly for a short period, then stretch much more slowly and
regularly for the balance of their life. The method described in
the previous paragraph is very useful in providing the one-time
high initial tension, and then sustaining the optimum tensioning
torque for the balance of the life of the belt with a minimum
number of adjustments. The method is especially useful for original
equipment manufacturers who seek belt life that matches the life of
the equipment.
The angle markings 14 on arm 11 are almost imperative on tensioners
that are used with drives that either have non-linear
torque/tension relationships, or lack definition of the region of
linearity. In many drives, sufficient analysis has probably been
done to identify satisfactory torque/tension relationships, and
only two angle markings, such as shown in FIG. 7, need be provided
to identify the end conditions. The end conditions and markings
indicate:
1. That a newly installed belt is long enough to provide acceptable
tension when the specified torque value is applied.
a. If the belt is too short, excessive tension will normally be
encountered unless the condition has been analyzed to meet the high
initial tension requirements mentioned in the previous paragraph.
Such an analysis should be within the capability of a highly
skilled analyst given enough input data.
b. If the belt is too long, useful belt life can be lost because
the user will see the extra length as an indication of belt stretch
which is an indication of diminished remaining life.
2. That belt stretch has met the belt manufacturer's recommended
value for remaining life, or that the belt is at the limit of the
specified region of torque/tension linearity.
The compliant, friction set, adjustable tensioner, as depicted by
FIGS. 3 and 4, is a dynamic tensioner that offers all of the
advantages of adjustable tensioners and many of the advantages of
automatic tensioners. Although there are similarities between the
devices illustrated in FIGS. 3 and 4 and torsion biased products
that are on the market today, the mathematical, construction, and
application techniques of the device of FIGS. 3 and 4 make them
unique both in concept and use.
In the embodiment of the invention illustrated in FIGS. 3 and 4,
the tensioner is generally indicated by the numeral 110 and
includes an arm 111 and a pulley and bearing assembly 112 secured
to one end thereof by pulley screw 113. It will be noted that the
end of the arm 111 which carries the pulley is bifurcated in this
embodiment.
As is the case with the embodiment of FIGS. 1 and 2, the tensioner
is attached to user mount 120 by lock screw 130 and pivot screw
140. There are, however, significant structural differences.
Thus, lock screw 130 passes through an arcuate slot 111a in an end
of arm 111 and a torque plate 116 and is then received in a
threaded bore 121 in user mount 120. The shank of lock screw 130 is
received in a spacer 131, as can be seen in FIG. 4.
The arm 111 also carries an elastomeric bushing through which the
pivot screw projects. It will be noted that torque plate 116 has an
upwardly extending cylindrical extension 116a which is surrounded
by this bushing. The bushing includes an outer sleeve 117, and
inner sleeve 119, adjacent the cylindrical projection 116a of
torque plate 116 and bonded or otherwise affixed thereto, and an
elastomeric sleeve 118 therebetween.
As is the case in FIGS. 1 and 2, the pivot screw 140 projects
beyond the lower surface 120a of user mount 120 and receives
Belleville spring 142 and clamping nut 141. Also, the shank of
pivot screw 140 is knurled as at 140a so as to prevent inner sleeve
119 from rotating with respect to the pivot screw.
In this family of tensioners, the tensioning torque is set by
tightening the clamping nut 141 down on the Belleville spring 142
until the friction torque, as measured by a torquing device that
rotates the pivot screw 140, is of a magnitude that will provide
the desired belt tension when the belt acts against the pulley and
bearing assembly 112, just as with the tensioner of FIGS. 1 and 2.
However, the compliant device places the elastomeric torsion
bushing 118 between the torque plate 116 to which the pivot screw
140 is fixed and the tensioner arm 111 to which the pulley and
bearing assembly 112 is attached, and through which the belt
reaction force must act. The elastomeric torsion bushing consists,
as noted, of the outer sleeve 117, the inner sleeve 119, and the
elastomer 118 which is bonded between the two. This torsion bushing
performs a number of functions, the most important of which
are:
1. It permits some rotation along with an appropriate restoring
torque between the tensioner arm 111 and the torque plate 116 to
minimize the effect on belt tension caused by variations in load on
the pulley and in belt length.
2. It prevents rubbing of parts at the tensioner pivot axis, and
consequently eliminates mechanical wear at this crucial point.
3. It provides some viscous damping because of the high hysteresis
properties of most elastomeric materials.
This action of the torsion bushing, in conjunction with the
precision initial torque setting of the compliant, friction set,
adjustable tensioner, permits the device to perform most of the
functions of automatic tensioners. This statement is based upon a
rather extensive computer-aided investigation of serpentine drives
in which, within cycle variations in belt tension due to torsional
vibrations, acceleration loads, component loads, and tensioner
damping were considered. Although the adjustable tensioner did not
compensate as well for long-term belt stretch, it was generally
superior in its response to oscillating conditions and could
generally respond as well or better to variations in dynamic load
due to prime mover acceleration and variations in driven component
loads. Consequently, it offers significant advantages in
maintaining tension under a wide range of conditions while
eliminating the serious problem of pivot wear in systems that will
receive enough routine maintenance to insure the simple tensioning
procedure previously described. This procedure may amount to
loosening the lock screw 130, rotating the tensioner arm 111 into
the belt until it springs back, and re-tightening the lock screw
130 once every 20,000 or 50,000 miles in some automotive
applications, or the equivalent in stationary and marine
applications.
Angle markings 14 and 114 are shown in FIGS. 1 and 3; however, they
are really needed only to denote minimum, maximum, and possibly
intermediate arm angles that are associated with key belt lengths.
These angles are actually relationships between the user mounts 20
and 120 and the tensioner arms 11 and 111. Relative motion between
the torque plate 116 and the tensioner arm 111, or the torque plate
116 and the lock screw 130, complicates using these relationships
as indicators of arm angle, but they can be used satisfactorily if
enough is known about the stress/strain relationships of the
torsion bushing. In general, however, either an indicator that is
functionally similar to the pointer 15 shown in FIG. 1, or markings
on the tensioner arm 111 shown in FIG. 3 that can be related to the
lock screw 130, will suffice. The markings can be as shown by angle
markings 14, or simply marks (not shown) on the outside of the
tensioner arm 111.
The particularly significant features of the compliant, friction
set, adjustable tensioner of FIGS. 3 and 4 are:
1. The torque plate which allows the setting torque supplied by the
friction mechanism to be applied directly to the torsion bushing
which transmits that precise amount of torque to the tensioner body
to provide the target tension in the belt.
2. The mechanism to set predetermined tensioning torques that will
insure the proper belt tension.
3. The various arrangements that are illustrated to exploit the
capabilities of the device.
Thus, FIGS. 3 and 4 depict a compliant, friction set, adjustable
tensioner that has a simply supported pulley and bearing assembly
112 mounted between the bifurcated arms of the tensioner arm 111,
as previously noted. It should be noted that the pulley and bearing
assembly 112 are mounted symmetrically to the torsion bushing to
help insure that the line of action of the belt reaction force
loads the torsion bushing symmetrically. The symmetric loading is
generally necessary to insure that the compliant torsion bushing
does not deflect and cause the belt to become misaligned from the
belt path that is established by the two pulleys that straddle the
tensioner. This requirement holds true for all tensioner
configurations.
Still referring to FIGS. 3 and 4, the mounting that is shown is
typical of an actual application. Generally, the tensioner
requirements are determined prior to installation. The key elements
of this determination are the location of the tensioner in the
system to provide the constant torque/tension relationship and the
amount of torque necessary to yield the desired tension. When the
tensioner is originally installed, the clamping nut 141 is
tightened down on the Belleville spring 142 until the friction
torque applied to the pivot screw 140 to rotate the unrestrained
torque plate 116 and tensioner arm 111 together is equal to the
previously determined torque that will provide the target tension.
This is a requirement for all configurations, and when this is
done, the tensioner is ready to be used to apply tension to the
belt.
The next step is to loosen the lock screw 130 and rotate the torque
plate 116 clockwise until the spring back of the tensioner arm 111
is observed after the setting torque is removed. Observation of
these two components will reveal that there is relative motion
between them as the pulley and bearing assembly 112 contacts the
belt, torque continues to be applied to the torque plate 116, and
relative rotation between the inner sleeve 119 and the outer sleeve
117 occurs as shear stresses are generated in the elastomer 118 of
the torsion bushing. The shear stresses are sustained because the
pivot screw 140 is effectively fixed to the inner sleeve 119, and
the outer sleeve 117 is effectively fixed to the tensioner arm 111.
Although transmission of the setting torque through the pivot screw
140 is not required, it is generally convenient.
The lock screw 130 is next tightened down to clamp the torque plate
116 between the spacer 131 and the user mount 120 so that no
rotation of the torque plate 116 can occur until the lock screw 130
is intentionally loosened during some subsequent operation. The
spacer 131 is long enough to insure that the tensioner arm 111 is
not clamped to the torque plate 116 so that the tensioner arm 111
can rotate to respond to varying belt tension and length
conditions. When the tensioner is viewed in plan, the torque plate
116 and the tensioner arm 111 will both be rotated clockwise, but
the torque plate 116 will have rotated further. Consequently, the
lock screw 130 will no longer appear to be symmetrically located in
the arcuate cutout 111a in the tensioner arm 111 because the
tensioner arm 111 will be displaced so the bottom edge of the
cutout will be nearer the screw. The spring rate of the torsion
bushing will have been chosen so that the full anticipated torque
of the system will not cause the tensioner arm 111 to touch the
spacer 131, except possibly when it is desired to over-tension the
belt in keeping with the manufacturer's recommendations when the
belt is new. In such a case, the tensioner arm 111 can stop against
the spacer 131 and some or all of the excess tensioning torque will
be absorbed by the lock screw 130 to avoid over-stressing the
torsion bushing.
When the tensioner is positioned and torqued as just described, it
is ready to perform the tensioning task for which it was intended,
and need only receive periodic adjustment to maintain suitable
tension for the life of the belt. This adjustment, as before,
consists only of loosening the lock screw 130, rotating the torque
plate 116 by turning the pivot screw 140 until the tensioner arm
111 springs back when the torque is removed, and finally
re-tightening the lock screw 130.
Some contend that an automatic belt tensioner must be positioned in
the belt strand of a multiple pulley system, such as a serpentine
drive that has the dynamic tension requirement that most nearly
approaches the static tension of the system. Actually, the static
tension of the conventional locked center system has little real
meaning to a system with an automatic tensioner that has a suitable
response to tension oscillations. The nature of belt drives is such
that low tensions near the low tension side of the prime mover
generate the necessary higher tensions as each driven component
comes closer to the high tension side of the prime mover. This is
in keeping with the nature of belt and chain drives which is that
the tension is greater on the entering side of the prime mover that
pulls the belt or chain, and is at the lowest on the exiting side
of the prime mover. An additional natural characteristic is that
the maximum tension is the minimum tension plus the sum of the
tension increments required to drive each of the driven components
in the system. For example, if the tension in the belt as it exits
the prime mover pulley is 100 Newtons and there are five driven
components, two of which require tension differentials of 50
Newtons, one that requires a tension differential of 150 Newtons,
and two that require tension differentials of 75 Newtons, the
maximum tension in the belt as it enters the prime mover pulley
will be [100+(50+50+150+75+75)]=500 Newtons. When dealing with the
locked center tensioner of FIG. 1, it is necessary to set the
static tension in the system such that it is equal to or greater
than one half of the sum of the operating tensions that enter and
exit the prime mover so the static tension in each span of the
example is 300 Newtons when the system is at rest. Furthermore, the
tension in each span is the sum of the tension in the preceding
span plus the incremental tension about the last pulley. This is
demonstrated by the following table of incremental tension about
each pulley and the tension in each span of the example. The prime
mover pulley is Pulley0.
______________________________________ Span Pulley No. Tension No.
Tension ______________________________________ 0-1 100 1 50 1-2 150
2 50 2-3 200 3 150 3-4 350 4 75 4-5 425 5 75 5-0 500 0 400
______________________________________
Keeping in mind that the incremental tensions about each pulley are
requirements of the driven components and that the capability of
the belt/pulley interface is a separate engineering problem, and
the belt mechanics discussed in the previous paragraph, it is
obvious that the tensions in the operating system are much
different in the non-rotating situation than in the rotating
situation. In review, the tension in each strand is the static
tension (300N in the example) in the non-rotating situation and
there is no tension variation as the belt rounds the pulley. In the
rotating system, the tension generally varies from strand to strand
by the amount of the incremental tension in each pulley that
precedes the strand. It becomes intuitively obvious that, as long
as the tension in any single strand is kept at the operating
tension required in that strand, the drive system will perform
correctly. This last statement is one of the fundamental keys to
successful automatic tensioners, although it appears to be lacking
in the patent literature. This statement is true even though the
non-rotating static tension is only a fraction of the static
tension required for the fixed center system. If this is extended
to the example, and we consider the locked center condition versus
an automatic tensioner in Strand 1-2, we have the following static
tensions in the non-rotating condition:
Locked Center Static Tension=300N.
Automatic Tensioner Static Tension=150N.
Although the example and the text consider only a simplified
multiple pulley drive system, they identify one of the primary
advantages of and one of the most important inherent
characteristics of dynamic tensioners.
There are two key features that any successful spring operated,
pivotal arm automatic tensioner must possess, and that is the
ability to provide the proper torque to maintain the desired belt
tension over a broad range of belt length conditions. Given the
inherent decay of spring torque with angular displacement, the
tensioner must be positioned within the system to match its torque
output with the tension requirements of the system. This is often
done by specifying (1) a nominal torque at a reference angular
displacement, and (2) a spring rate that causes spring torque decay
to almost exactly match the requirements of the system as the
tensioner is (3) positioned in the system. There are three very
important concepts here that have direct analogs in the compliant,
friction set, adjustable tensioner concept, and they are:
1. Nominal torque. This is directly analogous to the friction
torque set into the adjustable tensioner.
2. Spring Rate. A non-zero number for the automatic tensioner that
varies from application to application, but it is always zero for
the optimally applied, friction set adjustable tensioner,
regardless of the application.
3. Position. The automatic tensioner must be positioned so the
system requires tensioner torque that varies as the tensioner can
provide. The adjustable tensioner must be positioned so that the
system requires essentially constant tensioner torque
From the preceding, it is apparent to one skilled in the behavior
of multiple pulley belt systems that the compliant, friction set,
adjustable tensioner will provide all of the features of an
automatic tensioner, except its ability to compensate for belt
stretch without tension decay. The way around this shortcoming is
to set the friction torque a little high, allow the tension to
decay a little below the desired value, and re-tension the drive
periodically as previously described. The re-tensioning periods can
be quite protracted given the current availability of "high
modulus" belts. This practice is in keeping with accepted belt
tensioning practice. Furthermore, tests indicate that good friction
setting practice will provide practical "little high" and "little
low" values that are within the nominal tensioning tolerance of
spring driven automatic tensioners. Tensioning tolerances of spring
driven automatic tensioners are rarely less than +/-10%.
The last issue regarding the relationships between automatic and
compliant, friction set, adjustable tensioners are their ability to
perform as dynamic tensioners. Here, the term dynamic tensioner
refers to the tensioner's ability to respond to torsional
vibrations from all sources in the drive system. It is very
desirable for the tensioner to be able to respond to high
frequencies and maintain target tension. The more closely the
target tension is maintained, the more satisfactory the tensioner
is as a dynamic tensioner. The ability of the tensioner to perform
the dynamic function is related to the tensioner's natural
frequency which, in turn, is related to the spring rate and the
moment of inertia of the tensioner; in fact, the relationship is f
equals the square root of the quotient (k/j) where f is defined as
frequency, k is defined as spring rate, and j is defined as moment
of inertia. The following discussion of spring rate and moment of
inertia point out why the compliant, friction set adjustable
tensioner, has a higher natural frequency than the automatic
tensioner and, therefore, is a superior dynamic tensioner.
1. Spring Rate. The spring rate in this context refers to the
rapidity with which the tensioner will return to its original
position if temporarily displaced by a disruptive force which is
then removed. The spring rate of the Automatic Tensioner is the
spring rate that is used to compensate for changes in belt length.
Typical values of spring rate and torque in automotive applications
are k=150 Nm/Rev. when the torque is M=36 Nm. The compliant,
friction set, adjustable tensioner will characteristically be wound
about 1/15 revolution to arrive at nominal torque. Its restoring
torque to angle relationship would be roughly analogous to a spring
rate of k=T/R=36/(1/15)=540 Nm/Rev., or 3.6 times the rate of the
comparable Automatic Tensioner.
2. Moment of Inertia. The moment of inertia concerns the
distribution of mass about the axis of rotation. The major
contributor to the moment of inertia of all tensioners is the
pulley; however, replacing a 300 to 800 gram spring and extra
housing with 100 grams of torsion bushing is a contribution in a
region where contributions are very hard to find. A reduction of 5
to 10% in moment of inertia is realistic.
3. Natural Frequency. The preceding terms show significant
improvements in natural frequency. A conventional automatic
tensioner, as just described, will typically have a natural
frequency of 15 Hz., so the compliant, friction set, adjustable
tensioner, also just described, will have a natural frequency of 15
multiplied by the square root of [(540/150)*(1.075/1)]=29.5 Hz.,
almost double that of the automatic tensioner.
The preceding example demonstrates that the compliant, friction
set, adjustable tensioner is an inherently superior dynamic
tensioner.
All of the details discussed regarding FIGS. 3 and 4, belt
mechanics, dynamic tensioning, and comparisons with automatic
tensioners hold for the following discussion of some of the many
workable variations of the compliant, friction set, adjustable
tensioner.
FIG. 5 depicts a compliant, friction set, adjustable tensioner that
is intended to compensate for relatively little belt take-up (the
short tensioner arm length), minimum reverse bending stress in the
belt (large diameter pulley), and minimum room overall (no
tensioner body 211 protrusion for angle markings).
Again, the tensioner is generally indicated by the numeral 210 and
includes a tensioner arm or body 211 and a pulley and bearing
assembly 212 mounted on one end thereof.
A lock screw 230 and pivot screw 240 are also provided for
attachment of the tensioner 210 to the user mount 220. Also, as was
the case with the embodiment of FIGS. 3 and 4, a torque plate 216
having an upwardly extending cylindrical projection 216a is
disposed between the tensioner 210 and user mount 220.
Here, the pivot screw extends beneath the lower surface 220a of
user mount 220 and receives Belleville spring 242 and clamping nut
241.
An elastomeric bushing similar to that illustrated in FIG. 4 is
also provided and includes outer and inner sleeves 217 and 219 with
elastomer 218 therebetween. This bushing functions as previously
described. That is, the inner sleeve 219 of the bushing is pressed
over, bonded to or otherwise affixed to the cylindrical projection
216a. The knurled portion 240a of pivot screw 240 is pressed into
or otherwise affixed to the inside of projection 216a.
Torque for both friction setting and tensioning is best provided by
rotating the pivot screw 240 by inserting the appropriate wrench
through the hole 212a shown in the face of the pulley. General
manufacturing practice would require at least two holes in the
pulley to eliminate pulley imbalance during routine high speed
rotation.
FIG. 6 depicts a further version of the tensioner.
Again, the tensioner is generally indicated by the numeral 310 and
includes a tensioner arm or body 311 with a pulley and bearing
assembly 312 mounted on one end.
A lock screw 330 and pivot screw 340 are also provided for mounting
the tensioner on user mount 320. Also, as in the FIGS. 3 and 4
embodiment, a torque plate 316 is employed between the tensioner
arm or body 311 and the user mount 320.
Here, the Belleville spring 342 bears against base plate 350 when
clamping nut 341 is tightened.
In this configuration, the base plate 350 with studs 351 and 352 is
provided to permit a tensioner that has had the friction torque
adjusted prior to installation in the final application. After the
friction torque has been set, the pivot screw 340 and the studs 351
and 352 are inserted in the hole pattern provided by the user in
the user mount 320, and nuts 351a and 352a are added, as required,
to retain the tensioner. The installer then simply loosens the lock
screw 330 and rotates the tensioner into the belt until spring back
is observed, or a torque wrench placed on the flanged nut 343 is
torqued to the proper value. The lock pin 344 in the flanged nut
343 serves the dual purpose of preventing users from having access
to the clamping nut 341 and resetting the preset friction torque,
and to insure that the flanged nut 343 does not rotate during field
use. Such an arrangement is especially useful when trained
operators in a factory with precision tools and extensive equipment
can set the friction torque very accurately and rapidly. Relatively
untrained operators and users can thus be prevented from changing
the friction torque but can still apply the tensioner without
worrying about setting measurements. All of this leads to reduced
manufacturing and warranty costs for the manufacturer who uses the
tensioners on his end product.
Relatively minor modifications to FIG. 6 would place the hex head
of the pivot screw 340 on the other end of the tensioner and permit
screws that were threaded into the user mount 320 to hold the base
plate and the tensioner in position. This would permit access to
the tensioner from the same side of the user mount 320 as the
tensioner is mounted to provide easy access for tensioning in some
cases, and deny access to the clamping nut 341 to operators and the
end user. Conversely, the lock screw 330 could be tightened by
placing a nut on the same side of the user mount 320 as the flanged
nut 343 and the stud nuts 351a and 352a as shown.
FIGS. 7 and 8 show how markings 411b can be made on the outside of
the tensioner body 411 when no scale and/or pointer is provided,
and illustrate an arrangement for permitting friction setting,
locking, and tension setting from the same side of the tensioner.
This is very useful when the tensioner must be mounted against
existing fixed surfaces, or in new designs where access to both
sides of the user mount cannot be readily provided. It will be
noted that in all of the modifications illustrated in FIGS. 3
through 8, the torsion bushing is configured symmetrically to the
projected face of the pulley to insure that the pulley remains true
to the nominal belt path.
In FIGS. 7 and 8, the tensioner is generally indicated by the
numeral 410 and includes the tensioner arm or body 411 and the
pulley and bearing assembly 412.
Similarly, lock screw 430 and pivot or setting screw 445 are
provided for attachment to user mount 420. In this instance pivot
screw 440, which has a socket head 445, is merely threaded into
user mount 420 and secured by a mechanical or chemical locking
assist. A hex head shaft 446 is also provided. This shaft 446
surrounds screw 440 and is knurled at 446a for engagement with the
torque plate 416 and ultimately the bushing.
In that regard, the bushing includes inner and outer sleeves 417
and 419 and an elastomeric sleeve 418 therebetween and the torque
plate 416 includes an upwardly directed cylindrical projection 416a
which engages the knurled area 446a on the hex head shaft 446 and
is affixed to the inner sleeve 417 of the bushing.
The Belleville spring 442 and tension setting hexagon head 446b are
received on the side of the tensioner opposite torque plate 416 and
user mount 420 in this instance. The socket head pivot screw 445 is
used for setting friction torque and it will be noted that the
friction means, in the form of spring 442, will be trapped between
the head 445 and the head 446b.
The unique features of these devices are:
1. Using friction to provide the predetermined torque that matches
the system requirements for target tension at any point, or over a
specified range with specified nominal accuracy.
2. Using an undefined amount of tensioner spring back after the
setting torque is removed as an indication that enough setting
torque has been provided to eliminate the need for measuring
devices and permitting precise results to be derived from very
imprecise inputs.
3. Permitting the friction set device to be initially
over-tensioned to compensate for initial excessive belt stretch,
but still provide the correct tensioning torque in subsequent
tension settings.
4. Using the combination of predetermined and preset tension in
conjunction with the compliant torsion bushing to provide a
superior and predictable dynamic tensioner.
While a full and complete description of the invention has been set
forth in accordance with the dictates of the Patent Statutes, it
should be understood that modifications can be resorted to without
departing from the spirit hereof or the scope of the appended
claims.
Thus, in several modifications of the invention illustrated herein,
elastomeric bushings have been illustrated and described. It will
be understood that other compliant means such as springs could be
substituted. While bushings are thought to give superior dynamic
response because of their relatively high rates and freedom from
sliding surfaces, in suitable circumstances springs may serve
equally well.
* * * * *